The speed of movement of a hydraulically driven working member of a machine depends on the cross-sectional area of the principal narrowed orifices of the system and on the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to eliminate one of those variables, pressure drop. These systems include sense lines which transmit the pressure at one or more workports to the input of a variable displacement hydraulic pump which provides pressurized hydraulic fluid to actuators which drive working members of the machine. The resulting self adjustment of the pump output provides an approximately constant pressure drop across a control orifice whose cross-sectional area can be controlled by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of movement of the working member is determined only by the cross-sectional area of the orifice. One such system is disclosed in U.S. Pat. No. 4,693,272 issued to Wilke on Sep. 15, 1987, the disclosure of which is incorporated by reference.
Because in such a system the control valves and the hydraulic pump are normally not immediately adjacent to each other, the changing load pressure information must be transmitted to the remote pump input through hoses or other conduits which can be relatively long. Some oil tends to drain out of these conduits while the machine is in a stopped, neutral state. When the operator again calls for motion, these conduits must refill before the pressure compensation system can be fully effective. Because of the length of these conduits, the response of the pump may lag, and a slight dipping of the loads can occur. These may be referred to as the "lag time" and "start-up dipping" problems.
In some types of such systems, the "bottoming out" of a piston driving a load could cause the entire system to "hang up". This could occur in such systems which used the highest of the workport pressures to motivate the pressure compensation system. The bottomed out load would be the highest workport pressure; the pump could not provide a higher pressure; and thus there would no longer be a pressure drop across the control orifice. As a remedy, such systems may include a pressure relief valve in a load sensing circuit of the hydraulic control system. In the bottomed out situation, it would open to drop the sensed pressure to the load sense relief pressure, and this would allow the pump to provide a pressure drop across the control orifice.
While this solution is effective, it could have an undesirable side effect in systems which use a pressure compensating check valve as part of the means of holding substantially constant the pressure drop across the control orifice. The pressure relief valve could open even when no piston was bottomed out if a workport pressure exceeded the set point of the load sense relief valve. In that case, some fluid could flow back from the workport, backwards through the pressure compensating check valve, and into the pump chamber. As a result, the load could dip. This may be referred to as the "backflow" problem.
For the foregoing reasons, there is need for means to reduce or eliminate the problems of lag time, start-up dipping and backflow in some applications.